1. Field of Invention
This invention relates to the polarization of light from a tapered light pipe (TLP) in a projection system.
2. Description of the Related Art
Polarized light may be used to illuminate LCD imagers in LCD-type projection displays. LCD imagers may be, e.g. transmissive or reflective. Light input to the LCD imager may be polarized such that when the LCD pixels are modulated, the polarization of the selected pixels may be changed, and when the light output from the imager is analyzed by another polarizer, the selected pixels are darkened. When the pixels are modulated with the desired information, the information will be projected onto the screen.
When light is polarized, however, only half of the light will be of the correct polarization. The other half will be incorrectly polarized. Some of the incorrectly polarized light can be recovered by rotating the polarization with a certain efficiency, giving an overall efficiency of over 50%. Some of the incorrectly polarized light may be recovered, e.g., by passing it through a half-wave plate, after which it may be re-combined with the correctly polarized light.
In one polarization technique, as shown in FIG. 1a, a parallel beam of light 102 from a parabolic reflector may be focused into multiple beams 104 by using an array of lenses 106 called a fly eye lens. Each of the beams 104 may be refocused by another array of lenses 107 onto an array of polarizing beam splitters (PBS) 108. The PBS is a one dimensional array with stripes. A cell of PBS array 108 is shown in detail in FIG. 1b. A coating 108a on PBS 108 separates the input beam 104 into the parallel and perpendicular beams of light 111, 112. Beam 111 is redirected to the output direction by reflector 108b. The polarization of beam 111 is rotated by a half-wave plate 108c such that the exit beam 110 has the same polarization as beam 112.
In another polarization technique, as shown in FIG. 2, an elliptical reflector 202 may be used to focus light. In such a focused light system a straight light pipe (SLP) 204 is often used to collect and homogenize the beam profile, as described in U.S. Pat. No. 6,139,157, the disclosure of which is incorporated by reference. Multiple images 206 of the focused light are formed when viewed from the output of the SLP 204 due to the multiple reflections of the focused light by the sidewalls of the light pipe. The multiple images 206 may then be imaged onto PBS array 208 by lens 207. The two dimensional array of images formed by the SLP 204 is matched with PBS array 208 by having each row of the images image onto each stripe of the PBS array in a manner similar to the fly eye lens to produce light 210, 212 with the same polarization.
In optical systems where a 1:1 imaging system is needed for improved performance, a dual paraboloid reflector system may be used to focus the light. In a dual paraboloid reflector system, however, the focused light may have a very high numerical aperture (NA). A tapered light pipe (TLP) 318 may be used to transform a large NA to a smaller one in order to process the light further, as shown in FIG. 3.
Although multiple images of the focused light are formed when viewed from the output of the TLP 318 in a manner similar to the SLP, the reflections of the focused light do not form a flat surface at the output of the TLP. Rather, the focused light forms a curved surface 319 by the multiple reflections of the TLP. The degree of curvature of the surface 319 may be dependent on the angle of taper xcex1 (alpha) of the TLP 318. The taper angle xcex1 may be different in the horizontal and vertical directions to meet system requirements. Focusing this curved surface 319 to a flat surface PBS array may be expensive and difficult. There remains a need, therefore, for a system that can convert such a curved surface into a planar surface in order to perform polarization of light from a TLP.
An illumination engine for a projection display using a TLP is disclosed in one embodiment. The illumination engine includes a reflector having a first and second focal points, a source of electro-magnetic radiation disposed proximate to the first focal point of the reflector to emit rays of radiation that reflect from the reflector and converge substantially at the second focal point, a TLP having an input surface and an output surface, the input surface of the TLP disposed proximate to the second focal point to collect and transmit substantially all of the radiation, a SLP having an input surface and an output surface, the input surface of the SLP disposed proximate to the output surface of the TLP to collect and transmit substantially all of the radiation, a condenser lens disposed proximate to the output surface of the SLP to collect and transmit substantially all of the radiation, and a PBS disposed proximate to the condenser lens to collect and transmit substantially all of the radiation.
A method for using a TLP in a projection display is disclosed in a second embodiment. The method may be performed by positioning a source of electro-magnetic radiation at a first focal point of a reflector, producing rays of radiation by the source, reflecting the rays of radiation by the reflector toward a second focal point, positioning a TLP having an input surface and output surface so the input surface of the TLP is substantially proximate to the second focal point, collecting the rays of radiation at the input surface of the TLP, adjusting a numerical aperture of the radiation by passing the rays of radiation through the TLP, outputting rays of radiation from the output surface of the TLP, positioning a SLP having an input surface and output surface so the input surface of the SLP is substantially proximate to the output surface of the TLP, flattening a contour of the radiation by passing the rays of radiation through the SLP, and polarizing the radiation.
An illumination engine for a projection display using a TLP is disclosed in a third embodiment. The illumination engine includes a reflector having a first and second focal points, a source of electro-magnetic radiation disposed proximate to the first focal point of the reflector to emit rays of radiation that reflect from the reflector and converge substantially at the second focal point, a TLP having an input surface and an output surface, the input surface of the TLP disposed proximate to the second focal point to collect and transmit substantially all of the radiation, a lens having an input surface and an output surface, the input surface of the lens disposed proximate to the output surface of the TLP to collect and transmit substantially all of the radiation, a condenser lens disposed proximate to the output surface of the lens to collect and transmit substantially all of the radiation, and a PBS disposed proximate to the condenser lens to collect and transmit substantially all of the radiation. A lens or lens system is one example of a contoured delay element, as would be known to one of skill in the art.
A method for using a TLP in a projection display is disclosed in a fourth embodiment. The method may be performed by positioning a source of electro-magnetic radiation at a first focal point of a reflector, producing rays of radiation by the source, reflecting the rays of radiation by the reflector toward a second focal point, positioning a TLP having an input surface and output surface so the input surface of the TLP is substantially proximate to the second focal point, collecting the rays of radiation at the input surface of the TLP, adjusting a numerical aperture of the radiation by passing the rays of radiation through the TLP, outputting rays of radiation from the output surface of the TLP, positioning a lens having an input surface and output surface so the input surface of the lens is substantially proximate to the output surface of the TLP, flattening a contour of the radiation by passing the rays of radiation through the lens, and polarizing the radiation.